1. Field of the Invention
The present invention relates to underwater sound and more particularly to high power acoustic transducers (projectors) for applications such as non-lethal deterrence of terrorist swimmers and divers.
2. Brief Description of Prior Developments
Non-lethal swimmer and diver engagement is of increasing importance in today's threat environment because many potential terrorist targets are in areas accessible to recreational boaters or swimmers who may have no malevolent intent. The potential proximity of marine mammals also necessitates non-lethal methods.
Modern detection sonar systems are able to differentiate between marine mammals, large fish, swimmers and divers through their signature and track. They cannot, however, discern the intentions of a human in the water. Thus there is a need for a graduated system of engagement, beginning with audible warnings, sirens, etc. that should cause the casual intruder or marine life to turn away.
The later stages of engagement require a method that effectively incapacitates the intruder without lethal force, since there remains the possibility that they could be demonstrators, not terrorists. The ideal method would cause divers to surface where they could be dealt with by more conventional means.
The parameters of an ideal deterrent may be summarized to include effectiveness, high reliability, not being easily countered, using a graduated force level; non-lethality, affordability, and having size, weight and power source requirements appropriate to the application.
Short of developing the equivalent of a rubber bullet for underwater use, the candidates for non-lethal underwater deterrence are light and sound. Both can create psychophysical and/or physiological effects. Light, however, suffers from short propagation distances in the turbid water typical of many harbors and rivers. It is easily countered and does not work at all in the most turbid water.
High-intensity, low frequency sound is useful as a non-lethal means for deterring swimmers and divers who may be terrorists. The psychophysical acoustic interactions proposed to be exploited include annoyance/aversion (avoidance of a loud sound) and/or cognitive/functional task impairment (physical symptoms).
The physiological (based on frequency and sound pressure level (SPL) dependent thresholds) effects of low frequency sound are hearing (up to 160 dB SPL=minor effects) including auditory pain threshold ˜220 dB SPL, vestibular function (dizziness, rotation of visual field), and bronchopulmonary resonance (coughing, gagging, choking, pain).
It is difficult to defend against low frequency sound unless one is inside a rigid body such as a vehicle. Thus, resonance of the lungs is an ideal candidate for the deterrent method. Experimental evidence suggests that the nominal resonance frequency of the human lung is about 20 to 70 Hz and is depth dependent. The in situ damage threshold to mice and guinea pig lungs is reported to be about 180 dB SPL.
What is needed for an effective deterrent for underwater terrorists, therefore, is a relatively inexpensive, high power, low frequency source of underwater sound.